The present invention relates to a noise removal device for removing a noise mixed in a signal.
The present application claims priority from Japanese Application No. 2005-091876, the disclosure of which is incorporated herein by reference.
Generally, when an electronic device processes a signal, it is important to remove a noise from the signal.
For example, in a conventional FM radio receiver disclosed in Japanese Unexamined Patent Application Publication No. 10-75190, as shown in FIG. 1 of the patent application, a noise removal device is used to remove a noise which occurs once an over-modulated FM wave is FM-detected, and a signal processing such as a stereo demodulation is performed based on an FM detection signal (a composite signal) aimed at noise reduction.
Further, in a conventional vehicle radio receiver mounted on an automobile, a noise removal device is used to remove a noise such as an ignition noise mixed in an FM detection signal generated from an electronic device, and a signal processing such as a stereo demodulation is performed based on an FM detection signal aimed at noise reduction.
Now, with reference to FIG. 1 which is a block diagram and FIG. 2 which is a waveform chart, description will be given to explain the composition of a conventional and commonly used noise removal device provided in a vehicle radio receiver, also to explain a noise removal method. As shown, the conventional noise removal device comprises: a main path including a delay circuit 1 and a hold circuit 2; and a control path including a high pass filter 3, a smoothing circuit 4, a comparator 5, and a hold period adjusting circuit 6.
Once there is an input of an FM detection signal Sin containing a pulse-like noise Nz shown in FIG. 2(a), the high pass filter 3 in the control path will operate to inhibit certain signal component of the FM detection signal Sin, but allow the passing of the Noise Nz therethrough. Then, as shown in FIG. 2(b), a noise component Snz passed therethrough is smoothed by the smoothing circuit 4 to produce a smoothed DC signal Sdc which is used as a threshold value by the comparator 5 when the comparator 5 compares the DC signal Sdc with a noise component Snz, thereby generating a period detection signal Spd indicating an occurring period of the noise Nz shown in FIG. 2(c). Afterwards, the hold period adjusting circuit 6 will operate to finely adjust a time width of the period detection signal Spd in accordance with a predetermined fixed time, thereby generating a hold control signal Sh which is supplied to the hold circuit 2. On the other hand, with regard to the main path of the noise removal device, the delay circuit 1 will operate to make the FM detection signal Sin synchronized with the delay time of the control path before the FM detection signal Sin is inputted into the hold circuit 2. Then, as shown in FIG. 2(d), the hold circuit 2 will operate in synchronism with the hold control signal Sh to hold the noise Nz mixed in the FM detection signal Sin, thereby generating and thus outputting a noise-reduced FM detection signal Sout.
Thus, the conventional noise removal device is used to detect an accruing period of the noise Nz mixed in the FM detection signal Sin shown in FIG. 2(a), so as to generate the hold control signal Sh, and then, as shown in FIG. 2(d), to hold the Noise Nz in synchronism with the hold control signal Sh, thereby generating an FM detection signal Sout whose noise Nz has been reduced.
Moreover, the conventional FM radio receiver disclosed in Japanese Unexamined Patent Application Publication No. 10-75190 is used to extract a noise from an FM detection signal and then smoothen the extracted noise so as to produce a smoothed signal. Meanwhile, the smoothed signal is compared with the foregoing noise so as to generate a hold control signal indicating a noise occurring period. Then, the noise is held in synchronism with the hold control signal to generate a noise-reduced FM detection signal.
However, with regard to the noise removal devices provided in the aforementioned vehicle radio receiver and the FM radio receiver disclosed in Japanese Unexamined Patent Application Publication No. 10-75190, as shown in FIG. 2(b), a smoothed signal Sdc containing a smoothed noise occurred during a relatively long period is compared with each noise component to generate a hold control signal Sh in a manner such that a period during which a noise component Snz having a larger amplitude than the smoothed signal Sdc occurs will serve as a hold period. However, since the level (amplitude) of the smoothed signal Sdc will change in response to a change in the occurring density and the amplitude of the noise Nz, it was difficult to precisely detect the occurring period of each noise Nz.
Namely, as far as its nature is concerned, a noise is not something that occurs at certain density in time (in other words, frequency). In fact, each noise also changes independently from time to time in its intensity (amplitude) and waveform. For this reason, when the comparator 5 operates to detect an occurring period of a pulse-like noise occurring at certain instant in time, since it detects such an occurring period using a smoothed signal having a constant amplitude as a threshold value while being affected by the intensity and density of other noises occurring in a relatively long period, a change in the threshold level due to an influence from other noises makes it difficult to precisely detect the occurring period of the pulse-like noise, resulting in a problem that once a noise signal is held by the hold circuit 2, the noise will still be mixed in the FM detection signal Sout and this can cause part of the detection signal missing.